The invention relates to an electric conductor having an ignition device, which is in thermal contact with the conductor and is intended, if a limit temperature is exceeded, for igniting and destroying the conductor. One such conductor is known from German Patent Disclosure DE 1 96 20 204 A1.
This known conductor is intended for connecting one pole of a motor vehicle battery to the on-board electrical system of the vehicle and is intended to self-destruct, for example if as a consequence of an accident a short-circuit that could otherwise cause a vehicle fire occurs in the on board electrical system. It includes an electrical plug-type connector in a housing, in which the socket of the plug-type connector contains a priming cap, which if a short-circuit current flows through the conductor heats up and by its explosion forces the pin of the plug-type connector of the socket and thus interrupts the short-circuited current circuit.
Because of the transition resistance of the plug-type connector, the plug-type connector already heats up severely at even moderate currents. The extent of the heating depends on the quality of the contact between the pin and socket of the plug-type connector and is therefore variable for different examples of an identical plug-type connector. This is admittedly not critical when protecting against short-circuit currents, but it makes the known conductor unsuited to protecting consumers against electrical overload, where replicable ignition performance of the conductor is crucial. Even if it were possible for a consumer designed for high currents to be protected against excess currents with the aid of the known conductor, this would require using a very large-size plug-type connector, so as to keep its heating, at the currents required for operating the consumer, within the limits at which the ignition device will reliably not ignite. This would make the use of the known conductor expensive for securing high- current consumers, and if insufficient space for accommodating the plug-type connectors available, it would make it even impossible.
According to the invention, it is provided that a conductor of the type defined at the outset includes one or more strands extending continuously between the ends of the conductor, and that the ignition device is disposed on the conductor so as to split apart these strands in the event of ignition. This on the one hand makes a more-compact and substantially simpler structure of the conductor with the ignition device possible, because a conventional continuous cable can quite simply be considered for use as the conductor; second, a substantial voltage drop within the conductor is avoided by dispensing with the plug-type connector, which is especially desirable when consumers are supplied with high currents.
According to a preferred feature of the invention, the ignition device includes ignition material disposed annularly around the cable. This ignition material need not necessarily have a major explosive effect of the kind indispensable for severing the known conductor; it can suffice if it develops severe heat, which together with the heating of the conductor caused by the current flow causes the conductor to melt in the region of the ignition device, thus interrupting the flow of current to the consumer.
The ignition material can be surrounded by a capsule or housing that withstands the ignition. This housing has the effect on the one hand of protecting the surroundings of the conductor from severed, incandescently hot conductor material; it can also have the effect that the separated portions of the conductor are expelled from different openings of the capsule by the pressure of the hot material contained in the capsule, and thus the two portions are especially reliably separated from one another.
According to a second preferred feature, the ignition device is surrounded by the strands of the conductor. In that case, when the ignition device ignites, at least one of these strands is torn apart by the resultant explosive pressure, thus markedly reducing the conducting cross section of the conductor, and those strands that may not yet have torn apart then melt within minimal time.
To keep any influence on the conductor from heat sinks to which it is connected slight, the ignition device is preferably disposed approximately halfway along the length of the conductor.
The conductor is preferably a flexible stranded cable made up of many strands.
One important application of the conductor according to the invention is in automotive technology, in particular as a connection between a starter relay and a starter motor of a motor vehicle. The reason for this is that in modern motor vehicles, lighter and lighter starter motors are being used, which because of their low mass also have an only slight thermal capacity and can therefore easily overheat and become damaged if they are exposed to the starter current for too long. Furthermore, the heat that such an overloaded starter motor and/or its supply lines develop entails the risk of engine fires. Such overload situations can occur especially if a vehicle being driven by the starter motor alone must cover relatively long distances and in particular drive-up ramps, which is intrinsically a misuse that is not allowed yet often occurs when vehicles are being loaded, for instance onto railroad cars, trucks that transport cars, or ships. In such a case, the use of a conductor according to the invention as a connection between the starter relay and the starter motor assures effective protection of the starter motor and furthermore enables the user of the vehicle, who is often not the person who is misusing the starter as described, to have evidence of the misuse.
Further characteristics and advantages of the invention will become apparent from the ensuing description of exemplary embodiments with reference to the drawings.